Improving the Immunogenicity of the Mycobacterium bovis BCG Vaccine by Non-Genetic Bacterial Surface Decoration Using the Avidin-Biotin System

Robust Immune Response Induced by Schistosoma mansoni TSP-2 Antigen Coupled to Bacterial Outer Membrane Vesicles

Purpose

The use of adjuvants can significantly strengthen a vaccine’s efficacy. We sought to explore the immunization efficacy of bacterial outer membrane vesicles (OMVs) displaying the Schistosoma mansoni antigen, SmTSP-2, through a biotin-rhizavidin coupling approach. The rationale is to exploit the nanoparticulate structure and the adjuvant properties of OMVs to induce a robust antigen-specific immune response, in light of developing new vaccines against S. mansoni.

Materials and Methods

OMVs were obtained from Neisseria lactamica and conjugated with biotin. The recombinant SmTSP-2 in fusion with the biotin-binding protein rhizavidin (rRzvSmTSP-2) was produced in E. coli and coupled to biotinylated OMVs to generate an OMV complex displaying SmTSP-2 on the membrane surface (OMV:rSmTSP-2). Transmission electron microscopy (TEM) and dynamic light scattering analysis were used to determine particle charge and size. The immunogenicity of the vaccine complex was evaluated in C57BL/6 mice.

Results

The rRzvSmTSP-2 protein was successfully coupled to biotinylated OMVs and purified by size-exclusion chromatography. The OMV:rSmTSP-2 nanoparticles showed an average size of 200 nm, with zeta potential around – 28 mV. Mouse Bone Marrow Dendritic Cells were activated by the nanoparticles as determined by increased expression of the co-stimulatory molecules CD40 and CD86, and the proinflammatory cytokines (TNF-α, IL-6 and IL-12) or IL-10. Splenocytes of mice immunized with OMV:rSmTSP-2 nanoparticles reacted to an in vitro challenge with SmTSP-2 with an increased production of IL-6, IL-10 and IL-17 and displayed a higher number of CD4+ and CD8+ T lymphocytes expressing IFN-γ, IL-4 and IL-2, compared to mice immunized with the antigen alone. Immunization of mice with OMV:rSmTSP-2 induced a 100-fold increase in specific anti-SmTSP-2 IgG antibody titers, as compared to the group receiving the recombinant rSmTSP-2 protein alone or even co-administered with unconjugated OMV.

Conclusion

Our results demonstrate that the SmTSP-2 antigen coupled with OMVs is highly immunogenic in mice, supporting the potential effectiveness of this platform for improved antigen delivery in novel vaccine strategies.

A suitable and effective stepwise oxidative refolding procedure for highly-cationic tetrameric avidin in nucleic acid free conditions

Optimized conditions are needed to refold recombinant proteins from bacterial inclusion bodies into their biologically active conformations. In this study, we found two crucial requirements for efficient refolding of cationic tetrameric chicken avidin. The first step is to eliminate nucleic acid contaminants from the bacterial inclusion body. The electrostatic interactions between the remaining nucleic acids and proteins strongly enhanced protein aggregation during the refolding process. The cysteine specific reversible S-cationization procedure was successfully employed for large-scale preparation of nucleic acid free denatured protein without purification tag system. The second step is the intramolecular disulfide formation prior to refolding in dialysis removing denaturant. Disulfide intact monomeric avidin showed efficient formation of biologically active tetrameric conformation during the refolding process. Using this optimized refolding procedure, highly cationic avidin derivative designed as an intracellular delivery carrier of biotinylated protein was successfully prepared.

Expression of Exogenous Antigens in the Mycobacterium bovis BCG Vaccine via Non-genetic Surface Decoration with the Avidin-biotin System

Tuberculosis (TB) is a serious infectious disease and the only available vaccine M. bovis bacillus Calmette-Guérin (BCG) is safe and effective for protection against children's severe TB meningitis and some forms of disseminated TB, but fails to protect against pulmonary TB, which is the most prevalent form of the disease. Promising strategies to improve BCG currently rely either on its transformation with genes encoding immunodominant M. tuberculosis (Mtb)-specific antigens and/or complementation with genes encoding co-factors that would stimulate antigen presenting cells. Major limitations to these approaches include low efficiency, low stability, and the uncertain level of safety of expression vectors. In this study, we present an alternative approach to vaccine improvement, which consists of BCG complementation with exogenous proteins of interest on the surface of bacteria, rather than transformation with plasmids encoding corresponding genes. First, proteins of interest are expressed in fusion with monomeric avidin in standard E. coli expression systems and then used to decorate the surface of biotinylated BCG. Animal experiments using BCG surface decorated with surrogate ovalbumin antigen demonstrate that the modified bacterium is fully immunogenic and capable of inducing specific T cell responses. Altogether, the data presented here strongly support a novel and efficient method for reshaping the current BCG vaccine that replaces the laborious conventional approach of complementation with exogenous nucleic acids.

Autodisplay of an avidin with biotin-binding activity on the surface of Escherichia coli

OBJECTIVES

To display a recombinant avidin fused to the autotransporter ShdA to bind biotinylated molecules on the surface of Escherichia coli.

RESULTS

Two chimeric protein constructs containing avidin fused to the autotransporter ShdA were expressed on the surface of Escherichia coli DH5α. One fusion protein contained 476 amino acids of the ShdA α and β domains, whereas the second consisted of a 314 amino acid from α and truncated β domains. Protein production was verified by SDS-PAGE using an antibody to the molecular FLAG-tag. The surface display of the avidin-shdA fusion protein was confirmed by confocal microscopy and flow cytometry analysis, and the biotin-binding activity was evaluated by fluorescence microscopy and flow cytometry using biotin-4-fluorescein and biotinylated-ovalbumin (OVA).

CONCLUSIONS

Expression of a recombinant avidin with biotin-binding activity on the surface of E. coli was achieved using the autotransporter ShdA. This system is an alternative to bind biotinylated molecules to E. coli.

Expression of Cathepsin S in BCG converts it into a pro-apoptotic and highly immunogenic strain

BACKGROUND

BCG vaccine, introduced almost 100years ago, is the only option to prevent TB disease. It effectively protects newborns from meningeal TB but fails to prevent adult pulmonary TB. TB kills 1.3million people annually in areas where BCG vaccination is widely practiced. Thus, more effective TB vaccines are urgently needed. Others and we have shown that BCG mimics features of virulent M. tuberculosis, in particular attenuation of essential macrophage functions such as phagosome maturation and antigen presentation. One of these studies revealed that defect in antigen presentation is largely due to down-regulation of the cysteine protease Cathepsin S (CatS), which prevents MHC II molecule maturation and proper antigen peptide loading. Recent studies also suggested a potential role for cysteine proteases in the regulation of apoptosis, a key cellular process used by the macrophage to (i) contain and process ingested bacteria and (ii) facilitate cross-talk antigen presentation between the macrophage and dendritic cells.

METHOD

To reverse the phenotype of vaccine-mediated macrophage attenuation, we engineered a novel BCG strain that expresses and secretes active CatS (rBCG-CatS) to examine its pro-apoptotic properties in vitro, and subsequently, immunogenicity in mice.

RESULTS

Transcriptomic profiling of macrophages infected with rBCG-CatS, but not BCG, revealed upregulation of key pro-apoptotic genes and downregulation of anti-apoptotic genes, which were further confirmed by RT-qPCR analyses of expression of selected genes. Macrophages infected with rBCG-CatS undergo apoptosis as indicated by increased levels of annexin V staining and intracellular caspase-3 cleavage. Consistent with these findings, mice vaccinated with rBCG-CatS showed increased antigen-specific CD4⁺ T-cell responses, as well as enhanced cytokine production and proliferation in CD4⁺ upon ex vivo re-stimulation.

CONCLUSION

Collectively, this study shows that a pro-apoptotic BCG strain alleviates adverse traits of the wild-type strain, resulting in a highly immunogenic TB vaccine.

The principles and applications of avidin-based nanoparticles in drug delivery and diagnosis

Avidin-biotin interaction is one of the strongest non-covalent interactions in the nature. Avidin and its analogues have therefore been extensively utilized as probes and affinity matrices for a wide variety of applications in biochemical assays, diagnosis, affinity purification, and drug delivery. Recently, there has been a growing interest in exploring this non-covalent interaction in nanoscale drug delivery systems for pharmaceutical agents, including small molecules, proteins, vaccines, monoclonal antibodies, and nucleic acids. Particularly, the ease of fabrication without losing the chemical and biological properties of the coupled moieties makes the avidin-biotin system a versatile platform for nanotechnology. In addition, avidin-based nanoparticles have been investigated as diagnostic systems for various tumors and surface antigens. In this review, we will highlight the various fabrication principles and biomedical applications of avidin-based nanoparticles in drug delivery and diagnosis. The structures and biochemical properties of avidin, biotin and their respective analogues will also be discussed.

Layer-by-layer nanocoating of live Bacille-Calmette-Guérin mycobacteria with poly(I:C) and chitosan enhances pro-inflammatory activation and bactericidal capacity in murine macrophages

Tuberculosis (TB) is a major disease burden globally causing more than 1.5 million deaths per year. The attenuated live vaccine strain Bacille Calmette-Guérin (BCG), although providing protection against childhood TB, is largely ineffective against adult pulmonary TB. A major aim therefore is to increase the potency of the BCG vaccine to generate stronger and more sustained immunity against TB. Here, we investigated the use of layer-by-layer (LbL) nanocoating of the surface of live BCG with several layers of polyinosinic-polycytidylic acid (poly(I:C)), a strong inducer of cell-mediated immunity, and the biodegradable polysaccharide chitosan to enhance BCG immunogenicity. Nanocoating of live BCG did not affect bacterial viability or growth in vitro but induced killing of the BCG in infected mouse bone marrow-derived macrophages and enhanced macrophage production of pro-inflammatory cytokines and expression of surface co-stimulatory molecules relative to uncoated BCG. In addition, poly(I:C) surface-coated BCG, but not BCG alone or together with soluble poly(I:C), induced high production of nitric oxide (NO) and IL-12. These results argue that BCG and surface absorbed poly(I:C) act in a synergistic manner to elicit pro-inflammatory macrophage activation. In conclusion, nanocoating of live BCG with the immunostimulatory agent poly(I:C) may be an appropriate strategy to enhance and modulate host responses to the BCG vaccine.